Wet oxidation is an effective method for lowering the chemical oxygen demand (COD) of many compounds. When applied to waste treatment processes, wet oxidation involves the aqueous phase oxidation of predominantly organic compounds at elevated temperatures and pressures. Temperatures of 127-300° C. and pressures of 0.5-20 MPa are generally employed [1].
The vast majority of known processes relate to the wet oxidation of municipal sewage sludge [1].
The known processes all operate using the same general principles. A liquid or slurry is introduced into an autoclave via a pumping mechanism, and reacted with an oxidant under pressures and temperatures in the regions noted above. The products of the reaction are then removed from the reactor, largely in the liquid phase, cooled, and separated outside of the reactor.
Oxygen is only sparingly soluble in water and its use as an oxidant requires the process to be operated at a pressure significantly above that of the vapour pressure of the liquid at the operating temperature, in order to minimise the loss of liquid inside the reactor. However, the use of high temperatures and pressures which necessitate the use of, for example: expensive alloys in the fabrication of reactors; costly valves; and significant safety measures; together with the long reaction times required, have limited the application of wet oxidation.
One approach which has been applied to overcome these obstacles is the use of an appropriate catalyst. If a catalyst is employed to increase the rate of reaction, a lower temperature, and correspondingly lower pressure, is generally required. Typical catalysts comprise transition metal ions, the most effective of which are generally copper, manganese and iron. Due to their toxicity to other forms of life, these ions are restricted in terms of their disposal into public waterways. Therefore, strict disposal regulations apply to catalysts such as those based on transition metals.
Accordingly, the treated product from a wet oxidation reactor must have the concentration of the catalyst reduced to levels that satisfy the regulatory requirements. This may be achieved by, for example: precipitation as the hydroxide; pH modification to allow removal as the oxide; or a range of osmotic and electrochemical methods. All are costly and may be difficult to operate, thus limiting the use of catalysts in known processes.
Moreover, a mixture of compounds will usually be present in the waste stream to be treated and this further limits the known processes. The individual compounds generally have different rates of reaction, and therefore require different reactor residence times either in batchwise or continuous processes. This means that either:    (a) the reactor residence time is limited to that of the compound within the mixture with the slowest rate of reaction, which generally dictates the use of a proportionally larger reactor; or    (b) the process is less efficient and achieves a lower reduction in, for example, COD, biological oxygen demand (BOD) and percent conversion.
In addition, the more stable compounds—which are correspondingly more difficult to oxidise—may remain in the output stream from the reactor, and it is these compounds which are more likely to have detrimental environmental consequences.
These disadvantages have hindered the development of wet oxidation as a method for treating waste. Accordingly, it is an object of the present invention to provide a wet oxidation process which goes some way to overcoming these limitations, or at least provides the public with a useful choice.